In a machine such as a gas turbine engine, which includes a compressor, a combustor and turbine, seals or seal assemblies are disposed at various locations to minimize air leakage or control air flow direction. For example, annular seal assemblies or seal rings attached to a compressor exit diffuser create a flow path between the diffuser and rotor disks. The diffuser has an annular configuration and is coaxially aligned with a longitudinal axis of the rotor. Compressed air exits the compressor through the diffuser and is dispersed so that some air is drawn into the combustor for driving the turbine. In addition, some air exiting the compressor via the diffuser flows across components for cooling components, such as a combustor transition duct and components in a first stage of the turbine. However, some air will inevitably leak at locations such as the interconnection of the diffuser and compressor.
Older turbine engine designs operated at temperatures that were below the thermo-mechanical limitations of the engine component. Accordingly, significant cooling of spaces between components, such as the space between the diffuser and rotor disks, was not a primary objective for sealing. The seals included standard labyrinth or brush seals whose primary goal was to minimize leakage. However, more recent turbine engine designs demand higher operating temperatures, which may include temperatures that exceed the thermo-mechanical limitations of the component materials. Thus, controlling air flow in areas of the turbine, which were not previously required for cooling purposes, have now become more critical to controlling component temperatures so that the turbine engine operates more efficiently.
A prior art seal assembly 10 shown schematically in FIG. 1 is operatively connected to frame members 12 of a diffuser 14 facing rotor disks 22. The seal assembly 10 has an annular configuration and includes two end flanges 16 and 18 and a mid-section seal 20. As described above, the seal assembly 10 is intended to control the air flow or circulation of across components for cooling. The components 16, 18 and 20 of the seal assembly 10 as well as the diffuser 14 are all composed of materials having the same or substantially the same coefficient of thermal expansion (“CTE”).
The diffuser 14 and the seal assembly 10 components (16, 18, 20) are composed of the same material and, therefore, have the same coefficient of thermal expansion as schematically represented in FIG. 1, the mid-section seal 20 is thinner than the end flanges 16, 18, meaning it has a small thermal mass and a higher heat transfer coefficient relative to the diffuser 14. The flange ends 16, 18 of the seal assembly 10 are constrained by the adjacent diffuser frame member 12 that heats up more slowly due to its higher thermal mass and lower heat transfer coefficient at that connection. Thus, during a transient operation, for example, when a turbine engine is run until it reaches a steady state of operation, the operating temperature increases. When the operating temperature of the engine reaches thermo-mechanical limitations of the seal assembly materials, the seal mid-section deforms radially outward relative to the longitudinal axis of the turbine rotor (not shown), in part because the ends 16, 18 are constrained by the frame member 12 of the diffuser 14. In addition, as a result of the rotation of the disks 22, a surface 24 of the disks 22 undergoes thermo-mechanical deformation radially toward the longitudinally axis of the rotor, thereby widening the gap between the seal mid-section 20 and the rotor disks 22. When the engine reaches a steady state of operation at elevated temperatures of 535° C. this variation in gap size between the components can create a pressure differential that may increase the volume of drawn from the diffuser into this gap area. Accordingly, less air discharged from the compressor is available for combustion, which directly affects the operating efficiency of the turbine engine.